1. Field of the Invention
The present invention relates to a vibration driven motor using ultrasonic vibration and, more particularly, to a vibration driven motor used in a camera, OA equipment, and the like.
2. Related Background Art
A conventional pencil-type vibration driven motor is arranged as shown in FIG. 2. More specifically, a vibration member comprising a vibrating body 1, electrode plates 2, piezo-electric elements 3, a pressing body 4, and a bolt 5 is fixed by an adhesive to a fixing member 10 at an anti-rotation portion 5a of the bolt 5.
A spring case 6 is fitted on a shaft portion 10a of the fixing member 10. An output gear 8 is axially supported on the spring case 6 via a bearing 9, and is meshed with a rotor 7.
Since a compression spring 11 is arranged in the spring case 6, as shown in FIG. 2, it urges the rotor 7 against the vibrating body 1 via the spring case 6, the bearing 9, and the gear 8. The rotation of the rotor 7 is frictionally transmitted to the output gear 8.
In order to increase the output of the vibration driven motor, and to drive the motor with high efficiency, it is most important to urge the rotor 7 against the vibrating body 1 in a proper direction at a proper pressure without causing a nonuniformity in the surface pressure.
However, the conventional motor suffers from the following three drawbacks when the arrangement described above is employed.
(1) Adverse influence caused by side pressure of the output gear:
The output gear 8 inevitably receives a radial force (side pressure) when it transmits the output to a transmission gear (not shown). More specifically, in the conventional arrangement, a side pressure is transmitted to the fixing member 10 via the gear 8, the bearing 9, and the spring case 6. At this time, a frictional force between the spring case 6 and the fixing member 10 becomes a force against the biasing force of the compression spring 11, and the biasing force of the compression spring 11 cannot be properly transmitted between the rotor 7 and the vibrating body 1. As a result, the compression force between the rotor 7 and the vibrating body 1 is decreased. When the gear output is large, i.e., the side pressure is large, a frictional force between the spring case 6 and the fixing member 10 becomes too large to ignore, and the output and efficiency of the motor are reduced.
(2) Surface pressure nonuniformity due to bending of the pin 5b:
In the conventional arrangement, the rotor 7 and the output gear 8 are arranged to have the bearing 9 as a fulcrum. Since the bearing 9 as the fulcrum is located at a position separated from a contact portion 7b of the rotor 7 with the vibrating body 1, when the pin 5b is bent, the rotor 7 and the output gear 8 pivot, thus becoming inclined and eccentric with respect to the vibrating body 1. In this case, a surface pressure nonuniformity between the rotor 7 and the vibrating body 1 occurs due to the inclination and eccentricity, resulting in a reduction in the output and efficiency of the motor.
(3) Surface pressure nonuniformity due to a reaction force of a output of gear 8:
FIGS. 3A and 3B are schematic views showing a conventional arrangement.
In FIG. 3A, R indicates the radius of a pitch circle of the gear 8, M indicates the output torque, F indicates the reaction force of the torque, which is given by F=M/R, r indicates the radius of a rotor contact surface, and L indicates the distance between the center of the gear tooth and a rotor contact surface. As shown in FIG. 3B, a surface pressure distribution P due to the reaction force of the gear output is given by: EQU P=F.multidot.L.multidot.sin .theta./.pi.r.sup.2 (1) EQU P.sub.max =F.multidot.L/.pi.r.sup.2 (2)
In the conventional arrangement, the gear 8 and the rotor 7 are considered integrated since they are frictionally coupled to each other. In addition, the distance L between the gear and the contact surface is large, and hence, the surface pressure P due to the reaction force of the gear output (from the above equations) is large, resulting in a large surface pressure nonuniformity.
Therefore, the output and efficiency of the motor are reduced.